Why do I hate the end of Solo Leveling so much?

This article contains spoilers for the "Solo Leveling" story. Discretion is advised to the reader.
After several years in serialization, "Solo Leveling," the webtoon that took the world by storm for its epic action sequences and fantastic art style, has come to an end. Sung Jinwoo, the protagonist of the franchise, was forced to fight against everything imaginable, reaching all the way to the gods of his universe. In the finale of "Solo Leveling", Sung Jinwoo faced his toughest battle to date against Antares, the Monarch of Destruction.
Sung Jinwoo, who was unwilling to allow more people to die, carried the burden of defending the entire world, but he was not alone: he had his entire powerful Shadow Army behind him. While for many the epic final battle of the webtoon "Solo Leveling" was a satisfying ending, others ended the series with conflicting emotions. The series has a legion of fans who call "Solo Leveling" the best story ever created, but even among them there are many who still question the way the story came to a close.

One of the biggest criticisms was the final battle against the Monarch of Destruction, Antares. For all intents and purposes, it started off as epic as "Solo Leveling" was used to. In a climactic moment, Sung Jinwoo used a special ability to momentarily powerless Antares' army of dragons, forcing them to take refuge on a desert island in Japan, away from everyone else.
What followed was a battle of Monarch against Monarch; an army of shadows against an army of dragons. For a moment, Sung Jinwoo was able to defend himself, but Antares soon cornered him and offered him a deal. He would spare the world and disappear if the two worked together to destroy the Rulers. However, Jinwoo realized his lies and knew that Antares would not hesitate to kill him when he had the chance. Immediately, Antares transformed into a huge dragon and resumed combat. Since Jinwoo's weapons couldn't penetrate his tough armor, he decided he needed to create his own armor, based on darkness and shadows.
It's definitely a symbolic moment to see glimpses of Ashborn, Jinwoo's father, and images of his past self as he dealt the final blow to Antares, using the same dagger as his father. Although the panels of this combat are clearly the best in the entire webtoon, the glory is diminished by the appearance of the Rulers at the last moment, as if it were a deus ex machina, and the death of Antares with a final attack. This rendered all efforts of Jinwoo, one of the main critics at the end of "Solo Leveling", futile.
Another major reason for criticism at the end of "Solo Leveling" is Jinwoo's decision to use the Reincarnation Cup. After Jinwoo's initial fight against the Rulers, they thanked him for saving humanity, but Jinwoo was not satisfied. Sure, Antares was gone, but the Monarchs hadn't been defeated forever. More importantly, she had lost many people she cared about: her father and Adam were the most recent losses, not to mention the trauma her sister, Jinah, suffered from being attacked at school. Although he didn't prove it, these deaths and the effects the war with the Monarchs had on his family had a huge impact on Jinwoo.

To make things right, Jinwoo decides to go back in time to save everyone and prevent the Gates from opening. The Cup of Reincarnation turned back time, bringing back the Monarchs who died, even Antares, and keeping their memories intact. Jinwoo locked himself in the dimensional gap with the rest of the shadows that still existed in the past. From there, at the end of "Solo Leveling" the aforementioned Cup was activated and Jinwoo returned to his school days.
The reason why this whole sequence of events was so controversial for fans was that it seemed like an artificial way to allow Jinwoo to fix everything and make it perfect in the end. All the events of the series were now inconsequential, especially since Jinwoo didn't show many personal conflicts at the end. This is part of a larger criticism of "Solo Leveling" in general, because it has always been reproached for the characters being flat and superficial. However, since "Solo Leveling" had always been a power fantasy series from the beginning, Jinwoo gaining godlike powers that prevented anything from going wrong was perhaps always one of the cards available.
Another criticism related to the use of the Cup of Reincarnation came at the end, when Jinwoo defeated all the Monarchs and met Antares again. Unfortunately, the ending of the "Solo Leveling" webtoon didn't go into detail about how the fight unfolded, which was a bit disappointing, but not surprising. On the one hand, "Solo Leveling" had already offered fans a pretty epic fight in the previous chapters, so a second fight would prove repetitive. On the other hand, it is unlikely that the Rulers intervened a second time to make the battle between Jinwoo and Antares even more spectacular.
After turning back time with the Reincarnation Cup, they only spent two years in the real world during Jinwoo's fight against the Monarchs. However, in the dimensional gap, Jinwoo had been struggling for 27 years. Throughout that conflict, Jinwoo not only saved humanity, but also the Rulers. Had it not been for him, the Rulers would have been cornered by the strengthening of the Monarchs, continuing the endless cycle of war between Rulers and Monarchs for all eternity. Deciding to go back in time and single-handedly kill all the Monarchs, Jinwoo restored peace and freed the Rulers from their endless war.
However, the Rulers were not as grateful to Jinwoo as he had hoped. Ever since Norma Selner came into contact with the other world, her ability to predict the future had increased and the Rulers panicked when they heard that the God of Death would descend upon Earth with his army. This, of course, was demonstrated when Jinwoo appeared on Earth alone after finally single-handedly defeating the Monarchs.

In an iconic scene at the end of the webtoon, the Brightest Shard sat down with Jinwoo and gave him two options: walk away from Earth or be sealed. Fans have considered this pressure from the Rulers utterly hypocritical, considering all that Jinwoo did for them. After all, they are no different from the Monarchs, who tried to get rid of Ashborn because they feared his overwhelming power. However, this propensity to turn against those who have power over them is not new to the Rulers.
As deplorable as the idea of the Rulers turning against Jinwoo is, it was not lost on Jinwoo himself. He merely stared menacingly at the Shining Fragment, as if daring the Rulers to test him. Of course, the Rulers immediately back down, intimidated by Jinwoo's newfound power. In a way, Norma Selner's premonition wasn't entirely wrong, it just didn't happen as it seemed. Jinwoo was, of course, the God of Death, and his arrival marked the end of the world as everyone knew it. However, it also signified the beginning of a new world of peace.
Norma Selner's prophecy and Jinwoo's destructive power had an interesting effect on Jinwoo himself. Although he was angry with the Rulers, he began to question their existence: with their work done, was it already necessary? He went so far as to compare its great power to that of a nuclear weapon, capable of destroying the entire world if used without restraint.
What's more, the Reincarnation Cup not only turned back time, but also erased Jinwoo from everyone's memory. No one would ever remember the sacrifices he made to save the world. All the friends and allies he made as a world-famous Hunter would no longer remember who he was. It's a bit bittersweet to see Jinwoo reunited with his family and have a brief respite. When he confesses that he feels like he's had a nightmare, there's a hint of longing in his voice because, as strong as Jinwoo is, all he wants is to be a normal young man. There was a sense of vulnerability in Jinwoo's monologue as he seemed to plead with himself that postponing the battle with the Monarchs a little longer wouldn't hurt anyone.
Throughout "Solo Leveling", Jinwoo leveled up and became stronger because he had one purpose, which was to protect everyone. With the conflict finally over, he had to find a balance between his overwhelming power and having the humility not to use that power for the wrong reasons. However, maybe Jinwoo didn't need a heroic and grandiose reason to have power: maybe the reason could be as simple as wanting to live with his loved ones and protect them.

Despite the controversial ending of "Solo Leveling," it wasn't all bad. The series ended with Jinwoo being the strongest and founding his own family. It was a happy ending, even if there were some plot holes and inconsistencies. Luckily, new media from the "Solo Leveling" franchise continues to be released, which could help retroactively resolve some of the fan concerns.
Source: Reddit